1. Field of the Invention
The present invention relates to a technique of performing chromatic aberration correction of an image.
2. Description of the Related Art
Image capture apparatuses that employ imaging lenses, such as a digital camera, are used for various purposes. A light beam that has passed through an imaging lens has a refractive index in the imaging lens, which varies depending on its wavelength. Hence, even in light reflected by the same object, the distances from the imaging positions, on an image sensor, of light beams contained in the reflected light to the center of the optical axis on the image sensor differ depending on the wavelengths of these light beams. Such magnification chromatic aberration generates a color deviation, that is, a variation in imaging position in each individual color, so a color which is absent on the object under normal circumstances is generated in an image, thus leading to degradation in quality.
Since the number of pixels of an image sensor used increases each year, and the unit pixel size reduces, even a magnification chromatic aberration that is rarely problematic in the conventional techniques has become conspicuous. As a technique of correcting such a color deviation by image processing, a technique of acquiring the color deviation amount from an image to compensate for this color deviation amount has been proposed.
Japanese Patent Laid-Open Nos. 2000-299874 and 2006-020275, for example, disclose the following techniques. First, processing of obtaining the sum total of the differences in signal level between color components in each pixel after the position of image data formed by one color component is moved relative to that of image data formed by another color component is repeated on an edge portion. The moving amount of the position of image data formed by one color component relative to that of image data formed by another color component when the sum total of the differences in signal level between the color components minimizes is obtained to obtain a correction amount by which the color deviation amount minimizes.
However, in the method of acquiring a correction amount using the sum total of the differences in signal level between the color components as mentioned above, a precise correction amount for a color deviation due to magnification chromatic aberration often cannot be acquired on an edge portion on which not only magnification chromatic aberration but also a blur resulting from chromatic aberration on the axis is generated.
FIG. 9A shows changes in signal level of image data of color components of red (R), green (G), and blue (B) in a portion in which chromatic aberration on the axis is generated. Referring to FIG. 9A, due to the chromatic aberration on the axis, the correlation between the image data of the B color component and the image data of the G (or R) color component is lower than that between the image data of the R color component and the image data of the G color component. This is because the correlations between image data of a specific color component and image data of the remaining color components are lower in the portion in which chromatic aberration on the axis is generated than in the portion in which only magnification chromatic aberration is generated.
Hence, when a correction amount on an edge portion including chromatic aberration on the axis is obtained using the above-mentioned method, the color phase of the edge portion may change before and after correction.
FIG. 9B shows the state in which a correction amount is obtained using the sum total of the differences in signal level between the color components, and correction is performed using this correction amount, in the portion in which chromatic aberration on the axis is generated. Assume, for example, that a G plane serving as image data formed by a G color component and an R plane serving as image data formed by an R color component have no color deviation, and the G plane and a B plane serving as image data formed by a B color component have a color deviation, as shown in FIG. 9A. To minimize the difference in signal level between the color components on the B plane and the G (or R) plane, a region in which the signal level is higher on the B plane than on the G plane, and that in which the signal level is higher on the G plane than on the B plane mix with each other. Especially when the correlations of the B plane to the planes of the remaining color components are low, a region in which the difference in signal level between the B and G planes is large is present. For example, a portion corresponding to a hatched portion in FIG. 9A is in yellow, but a portion corresponding to a hatched portion in FIG. 9B is in another color, blue due to excessive correction, so the observer feels a great sense of discomfort upon a comparison between the color phases of the edge portion before and after correction. A method of relieving the observer's sense of discomfort by adjusting the color phase of the edge portion after correction is available, but it may deteriorate the original color structure of the object.